This section contains descriptions of server settings that cannot be changed at the session or query level.
These settings are stored in the `config.xml` file on the ClickHouse server.
Other settings are described in the “[Settings](../../operations/settings/index.md#session-settings-intro)” section.
Before studying the settings, read the [Configuration files](../../operations/configuration-files.md#configuration_files) section and note the use of substitutions (the `incl` and `optional` attributes).
Period in seconds for updating asynchronous metrics.
Type: UInt32
Default: 120
## asynchronous_metrics_update_period_s
Period in seconds for updating asynchronous metrics.
Type: UInt32
Default: 1
## background_buffer_flush_schedule_pool_size
The maximum number of threads that will be used for performing flush operations for Buffer-engine tables in the background.
Type: UInt64
Default: 16
## background_common_pool_size
The maximum number of threads that will be used for performing a variety of operations (mostly garbage collection) for \*MergeTree-engine tables in a background.
Type: UInt64
Default: 8
## background_distributed_schedule_pool_size
The maximum number of threads that will be used for executing distributed sends.
Type: UInt64
Default: 16
## background_fetches_pool_size
The maximum number of threads that will be used for fetching data parts from another replica for \*MergeTree-engine tables in a background.
Type: UInt64
Default: 8
## background_merges_mutations_concurrency_ratio
Sets a ratio between the number of threads and the number of background merges and mutations that can be executed concurrently. For example, if the ratio equals to 2 and
`background_pool_size` is set to 16 then ClickHouse can execute 32 background merges concurrently. This is possible, because background operations could be suspended and postponed. This is needed to give small merges more execution priority. You can only increase this ratio at runtime. To lower it you have to restart the server.
The same as for `background_pool_size` setting `background_merges_mutations_concurrency_ratio` could be applied from the `default` profile for backward compatibility.
Type: Float
Default: 2
## background_merges_mutations_scheduling_policy
The policy on how to perform a scheduling for background merges and mutations. Possible values are: `round_robin` and `shortest_task_first`.
## background_merges_mutations_scheduling_policy
Algorithm used to select next merge or mutation to be executed by background thread pool. Policy may be changed at runtime without server restart.
Could be applied from the `default` profile for backward compatibility.
Possible values:
- "round_robin" — Every concurrent merge and mutation is executed in round-robin order to ensure starvation-free operation. Smaller merges are completed faster than bigger ones just because they have fewer blocks to merge.
- "shortest_task_first" — Always execute smaller merge or mutation. Merges and mutations are assigned priorities based on their resulting size. Merges with smaller sizes are strictly preferred over bigger ones. This policy ensures the fastest possible merge of small parts but can lead to indefinite starvation of big merges in partitions heavily overloaded by INSERTs.
Type: String
Default: round_robin
## background_message_broker_schedule_pool_size
The maximum number of threads that will be used for executing background operations for message streaming.
Type: UInt64
Default: 16
## background_move_pool_size
The maximum number of threads that will be used for moving data parts to another disk or volume for *MergeTree-engine tables in a background.
Type: UInt64
Default: 8
## background_pool_size
Sets the number of threads performing background merges and mutations for tables with MergeTree engines. You can only increase the number of threads at runtime. To lower the number of threads you have to restart the server. By adjusting this setting, you manage CPU and disk load. Smaller pool size utilizes less CPU and disk resources, but background processes advance slower which might eventually impact query performance.
Before changing it, please also take a look at related MergeTree settings, such as `number_of_free_entries_in_pool_to_lower_max_size_of_merge` and `number_of_free_entries_in_pool_to_execute_mutation`.
Type: UInt64
Default: 16
## background_schedule_pool_size
The maximum number of threads that will be used for constantly executing some lightweight periodic operations for replicated tables, Kafka streaming, and DNS cache updates.
Type: UInt64
Default: 128
## backup_threads
The maximum number of threads to execute BACKUP requests.
Type: UInt64
Default: 16
## backups_io_thread_pool_queue_size
The maximum number of jobs that can be scheduled on the Backups IO Thread pool. It is recommended to keep this queue unlimited (0) due to the current S3 backup logic.
Type: UInt64
Default: 0
## cache_size_to_ram_max_ratio
Set cache size to RAM max ratio. Allows lowering the cache size on low-memory systems.
Type: Double
Default: 0.5
## concurrent_threads_soft_limit_num
The maximum number of query processing threads, excluding threads for retrieving data from remote servers, allowed to run all queries. This is not a hard limit. In case if the limit is reached the query will still get at least one thread to run. Query can upscale to desired number of threads during execution if more threads become available.
Zero means Unlimited.
Type: UInt64
Default: 0
## concurrent_threads_soft_limit_ratio_to_cores
Same as concurrent_threads_soft_limit_num, but with ratio to cores.
Type: UInt64
Default: 0
## default_database
Default database name.
Type: String
Default: default
## disable_internal_dns_cache
Disables the internal DNS cache. Recommended for operating ClickHouse in systems
with frequently changing infrastructure such as Kubernetes.
Size of cache for index marks. Zero means disabled.
Type: UInt64
Default: 0
## index_uncompressed_cache_size
Size of cache for uncompressed blocks of MergeTree indices. Zero means disabled.
Type: UInt64
Default: 0
## io_thread_pool_queue_size
Queue size for IO thread pool. Zero means unlimited.
Type: UInt64
Default: 10000
## mark_cache_policy
Mark cache policy name.
Type: String
Default: SLRU
## mark_cache_size
Size of cache for marks (index of MergeTree family of tables).
Type: UInt64
Default: 5368709120
## max_backup_bandwidth_for_server
The maximum read speed in bytes per second for all backups on server. Zero means unlimited.
Type: UInt64
Default: 0
## max_backups_io_thread_pool_free_size
If the number of **idle** threads in the Backups IO Thread pool exceeds `max_backup_io_thread_pool_free_size`, ClickHouse will release resources occupied by idling threads and decrease the pool size. Threads can be created again if necessary.
Type: UInt64
Default: 0
## max_backups_io_thread_pool_size
The maximum number of threads that would be used for IO operations for BACKUP queries
Limit on total number of concurrently executed queries. Zero means Unlimited. Note that limits on insert and select queries, and on the maximum number of queries for users must also be considered. See also max_concurrent_insert_queries, max_concurrent_select_queries, max_concurrent_queries_for_all_users. Zero means unlimited.
These settings can be modified at runtime and will take effect immediately. Queries that are already running will remain unchanged.
:::
Type: UInt64
Default: 0
## max_concurrent_select_queries
Limit on total number of concurrently select queries. Zero means Unlimited.
:::note
These settings can be modified at runtime and will take effect immediately. Queries that are already running will remain unchanged.
:::
Type: UInt64
Default: 0
## max_connections
Max server connections.
Type: Int32
Default: 1024
## max_io_thread_pool_free_size
Max free size for IO thread pool.
Type: UInt64
Default: 0
## max_io_thread_pool_size
The maximum number of threads that would be used for IO operations
Type: UInt64
Default: 100
## max_local_read_bandwidth_for_server
The maximum speed of local reads in bytes per second. Zero means unlimited.
Type: UInt64
Default: 0
## max_local_write_bandwidth_for_server
The maximum speed of local writes in bytes per second. Zero means unlimited.
Type: UInt64
Default: 0
## max_partition_size_to_drop
Restriction on dropping partitions.
If the size of a [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) table exceeds `max_partition_size_to_drop` (in bytes), you can’t drop a partition using a [DROP PARTITION](../../sql-reference/statements/alter/partition.md#drop-partitionpart) query.
This setting does not require a restart of the Clickhouse server to apply. Another way to disable the restriction is to create the `<clickhouse-path>/flags/force_drop_table` file.
Default value: 50 GB.
The value 0 means that you can drop partitions without any restrictions.
:::note
This limitation does not restrict drop table and truncate table, see [max_table_size_to_drop](#max-table-size-to-drop)
:::
## max_remote_read_network_bandwidth_for_server
The maximum speed of data exchange over the network in bytes per second for read. Zero means unlimited.
Type: UInt64
Default: 0
## max_remote_write_network_bandwidth_for_server
The maximum speed of data exchange over the network in bytes per second for write. Zero means unlimited.
Type: UInt64
Default: 0
## max_server_memory_usage
Limit on total memory usage. Zero means Unlimited.
The default `max_server_memory_usage` value is calculated as `memory_amount * max_server_memory_usage_to_ram_ratio`.
Type: UInt64
Default: 0
## max_server_memory_usage_to_ram_ratio
Same as max_server_memory_usage but in a ratio to physical RAM. Allows lowering the memory usage on low-memory systems. Zero means unlimited.
On hosts with low RAM and swap, you possibly need setting `max_server_memory_usage_to_ram_ratio` larger than 1.
Type: Double
Default: 0.9
## max_table_size_to_drop
Restriction on deleting tables.
If the size of a [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) table exceeds `max_table_size_to_drop` (in bytes), you can’t delete it using a [DROP](../../sql-reference/statements/drop.md) query or [TRUNCATE](../../sql-reference/statements/truncate.md) query.
This setting does not require a restart of the Clickhouse server to apply. Another way to disable the restriction is to create the `<clickhouse-path>/flags/force_drop_table` file.
Default value: 50 GB.
The value 0 means that you can delete all tables without any restrictions.
The maximum amount of storage that could be used for external aggregation, joins or sorting.
Queries that exceed this limit will fail with an exception. Zero means unlimited.
Also see `max_temporary_data_on_disk_size_for_user` and `max_temporary_data_on_disk_size_for_query`.
Type: UInt64
Default: 0
## max_thread_pool_free_size
If the number of **idle** threads in the Global Thread pool is greater than `max_thread_pool_free_size`, then ClickHouse releases resources occupied by some threads and the pool size is decreased. Threads can be created again if necessary.
Type: UInt64
Default: 1000
## max_thread_pool_size
The maximum number of threads that could be allocated from the OS and used for query execution and background operations.
Type: UInt64
Default: 10000
## mmap_cache_size
Sets the cache size (in bytes) for mapped files. This setting allows avoiding frequent open/close calls (which are very expensive due to consequent page faults), and to reuse mappings from several threads and queries. The setting value is the number of mapped regions (usually equal to the number of mapped files). The amount of data in mapped files can be monitored in the tables system.metrics and system.metric_log with the `MMappedFiles` and `MMappedFileBytes` metrics. Also, in system.asynchronous_metrics and system.asynchronous_metrics_log by the `MMapCacheCells` metric, and in system.events, system.processes, system.query_log, system.query_thread_log, system.query_views_log by the `CreatedReadBufferMMap`, `CreatedReadBufferMMapFailed`, `MMappedFileCacheHits`, `MMappedFileCacheMisses` events.
Note that the amount of data in mapped files does not consume memory directly and is not accounted for in query or server memory usage — because this memory can be discarded similar to the OS page cache. The cache is dropped (the files are closed) automatically on the removal of old parts in tables of the MergeTree family, also it can be dropped manually by the `SYSTEM DROP MMAP CACHE` query.
Type: UInt64
Default: 1000
## restore_threads
The maximum number of threads to execute RESTORE requests.
Type: UInt64
Default: 16
## show_addresses_in_stack_traces
If it is set true will show addresses in stack traces
Type: Bool
Default: 1
## shutdown_wait_unfinished_queries
If set true ClickHouse will wait for running queries finish before shutdown.
Type: Bool
Default: 0
## temporary_data_in_cache
With this option, temporary data will be stored in the cache for the particular disk.
In this section, you should specify the disk name with the type `cache`.
In that case, the cache and temporary data will share the same space, and the disk cache can be evicted to create temporary data.
:::note
Only one option can be used to configure temporary data storage: `tmp_path` ,`tmp_policy`, `temporary_data_in_cache`.
:::
**Example**
Both the cache for `local_disk`, and temporary data will be stored in `/tiny_local_cache` on the filesystem, managed by `tiny_local_cache`.
TThe maximum number of jobs that can be scheduled on the Global Thread pool. Increasing queue size leads to larger memory usage. It is recommended to keep this value equal to `max_thread_pool_size`. Zero means unlimited.
Type: UInt64
Default: 10000
## tmp_policy
Policy for storage with temporary data. Also see the MergeTree Table Engine documentation.
:::note
- Only one option can be used to configure temporary data storage: `tmp_path` ,`tmp_policy`, `temporary_data_in_cache`.
-`move_factor`, `keep_free_space_bytes`,`max_data_part_size_bytes` and are ignored.
- Policy should have exactly *one volume* with *local* disks.
:::
**Example**
When `/disk1` is full, temporary data will be stored on `/disk2`.
```xml
<clickhouse>
<storage_configuration>
<disks>
<disk1>
<path>/disk1/</path>
</disk1>
<disk2>
<path>/disk2/</path>
</disk2>
</disks>
<policies>
<!-- highlight-start -->
<tmp_two_disks>
<volumes>
<main>
<disk>disk1</disk>
<disk>disk2</disk>
</main>
</volumes>
</tmp_two_disks>
<!-- highlight-end -->
</policies>
</storage_configuration>
<!-- highlight-start -->
<tmp_policy>tmp_two_disks</tmp_policy>
<!-- highlight-end -->
</clickhouse>
```
Type: String
Default:
## uncompressed_cache_policy
Uncompressed cache policy name.
Type: String
Default: SLRU
## uncompressed_cache_size
Cache size (in bytes) for uncompressed data used by table engines from the MergeTree family. Zero means disabled.
There is one shared cache for the server. Memory is allocated on demand. The cache is used if the option use_uncompressed_cache is enabled.
The uncompressed cache is advantageous for very short queries in individual cases.
Configures a command to obtain a key to be used by [encryption codecs](../../sql-reference/statements/create/table.md#create-query-encryption-codecs). Key (or keys) should be written in environment variables or set in the configuration file.
Storing keys in the configuration file is not recommended. It isn't secure. You can move the keys into a separate config file on a secure disk and put a symlink to that config file to `config.d/` folder.
(see [database_catalog_unused_dir_hide_timeout_sec](../../operations/server-configuration-parameters/settings.md#database_catalog_unused_dir_hide_timeout_sec))
If `true`, then each dictionary is created on first use. If dictionary creation failed, the function that was using the dictionary throws an exception.
If `false`, all dictionaries are created when the server starts, if the dictionary or dictionaries are created too long or are created with errors, then the server boots without of these dictionaries and continues to try to create these dictionaries.
- interval – The interval for sending, in seconds.
- timeout – The timeout for sending data, in seconds.
- root_path – Prefix for keys.
- metrics – Sending data from the [system.metrics](../../operations/system-tables/metrics.md#system_tables-metrics) table.
- events – Sending deltas data accumulated for the time period from the [system.events](../../operations/system-tables/events.md#system_tables-events) table.
- events_cumulative – Sending cumulative data from the [system.events](../../operations/system-tables/events.md#system_tables-events) table.
- asynchronous_metrics – Sending data from the [system.asynchronous_metrics](../../operations/system-tables/asynchronous_metrics.md#system_tables-asynchronous_metrics) table.
Expired time for HSTS in seconds. The default value is 0 means clickhouse disabled HSTS. If you set a positive number, the HSTS will be enabled and the max-age is the number you set.
A username and a password used to connect to other servers during [replication](../../engines/table-engines/mergetree-family/replication.md). Also the server authenticates other replicas using these credentials. So, `interserver_http_credentials` must be the same for all replicas in a cluster.
-`allow_empty` — If `true`, then other replicas are allowed to connect without authentication even if credentials are set. If `false`, then connections without authentication are refused. Default value: `false`.
-`old` — Contains old `user` and `password` used during credential rotation. Several `old` sections can be specified.
ClickHouse supports dynamic interserver credentials rotation without stopping all replicas at the same time to update their configuration. Credentials can be changed in several steps.
To enable authentication, set `interserver_http_credentials.allow_empty` to `true` and add credentials. This allows connections with authentication and without it.
To change existing credentials, move the username and the password to `interserver_http_credentials.old` section and update `user` and `password` with new values. At this point the server uses new credentials to connect to other replicas and accepts connections with either new or old credentials.
Backlog (queue size of pending connections) of the listen socket.
Default value: `4096` (as in linux [5.4+](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=19f92a030ca6d772ab44b22ee6a01378a8cb32d4)).
So even if you have `TcpExtListenOverflows` (from `nstat`) non zero and this counter grows for ClickHouse server it does not mean that this value need to be increased, since:
-`log`– The log file. Contains all the entries according to `level`.
-`errorlog`– Error log file.
-`size`– Size of the file. Applies to `log` and `errorlog`. Once the file reaches `size`, ClickHouse archives and renames it, and creates a new log file in its place.
-`count`– The number of archived log files that ClickHouse stores.
-`console`– Send `log` and `errorlog` to the console instead of file. To enable, set to `1` or `true`.
-`stream_compress`– Compress `log` and `errorlog` with `lz4` stream compression. To enable, set to `1` or `true`.
- use_syslog — Required setting if you want to write to the syslog.
- address — The host\[:port\] of syslogd. If omitted, the local daemon is used.
- hostname — Optional. The name of the host that logs are sent from.
- facility — [The syslog facility keyword](https://en.wikipedia.org/wiki/Syslog#Facility) in uppercase letters with the “LOG_” prefix: (`LOG_USER`, `LOG_DAEMON`, `LOG_LOCAL3`, and so on).
Default value: `LOG_USER` if `address` is specified, `LOG_DAEMON` otherwise.
- format – Message format. Possible values: `bsd` and `syslog.`
The server will need access to the public Internet via IPv4 (at the time of writing IPv6 is not supported by Sentry) for this feature to be functioning properly.
-`enabled`– Boolean flag to enable the feature, `false` by default. Set to `true` to allow sending crash reports.
-`endpoint`– You can override the Sentry endpoint URL for sending crash reports. It can be either a separate Sentry account or your self-hosted Sentry instance. Use the [Sentry DSN](https://docs.sentry.io/error-reporting/quickstart/?platform=native#configure-the-sdk) syntax.
-`anonymize` - Avoid attaching the server hostname to the crash report.
-`http_proxy` - Configure HTTP proxy for sending crash reports.
-`debug` - Sets the Sentry client into debug mode.
-`tmp_path` - Filesystem path for temporary crash report state.
-`environment` - An arbitrary name of an environment in which the ClickHouse server is running. It will be mentioned in each crash report. The default value is `test` or `prod` depending on the version of ClickHouse.
For more information, see the section [Creating replicated tables](../../engines/table-engines/mergetree-family/replication.md#creating-replicated-tables).
If the size of a [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) table exceeds `max_table_size_to_drop` (in bytes), you can’t delete it using a [DROP](../../sql-reference/statements/drop.md) query or [TRUNCATE](../../sql-reference/statements/truncate.md) query.
This setting does not require a restart of the Clickhouse server to apply. Another way to disable the restriction is to create the `<clickhouse-path>/flags/force_drop_table` file.
If the size of a [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) table exceeds `max_partition_size_to_drop` (in bytes), you can’t drop a partition using a [DROP PARTITION](../../sql-reference/statements/alter/partition.md#drop-partitionpart) query.
This setting does not require a restart of the Clickhouse server to apply. Another way to disable the restriction is to create the `<clickhouse-path>/flags/force_drop_table` file.
Default value: 50 GB.
The value 0 means that you can drop partitions without any restrictions.
ClickHouse uses threads from the Global Thread pool to process queries. If there is no idle thread to process a query, then a new thread is created in the pool. `max_thread_pool_size` limits the maximum number of threads in the pool.
If the number of **idle** threads in the Global Thread pool is greater than `max_thread_pool_free_size`, then ClickHouse releases resources occupied by some threads and the pool size is decreased. Threads can be created again if necessary.
The maximum number of jobs that can be scheduled on the Global Thread pool. Increasing queue size leads to larger memory usage. It is recommended to keep this value equal to [max_thread_pool_size](#max-thread-pool-size).
ClickHouse uses threads from the IO Thread pool to do some IO operations (e.g. to interact with S3). `max_io_thread_pool_size` limits the maximum number of threads in the pool.
If the number of **idle** threads in the IO Thread pool exceeds `max_io_thread_pool_free_size`, ClickHouse will release resources occupied by idling threads and decrease the pool size. Threads can be created again if necessary.
ClickHouse uses threads from the Backups IO Thread pool to do S3 backup IO operations. `max_backups_io_thread_pool_size` limits the maximum number of threads in the pool.
If the number of **idle** threads in the Backups IO Thread pool exceeds `max_backup_io_thread_pool_free_size`, ClickHouse will release resources occupied by idling threads and decrease the pool size. Threads can be created again if necessary.
The maximum number of jobs that can be scheduled on the Backups IO Thread pool. It is recommended to keep this queue unlimited due to the current S3 backup logic.
Sets the number of threads performing background merges and mutations for tables with MergeTree engines. This setting is also could be applied at server startup from the `default` profile configuration for backward compatibility at the ClickHouse server start. You can only increase the number of threads at runtime. To lower the number of threads you have to restart the server. By adjusting this setting, you manage CPU and disk load. Smaller pool size utilizes less CPU and disk resources, but background processes advance slower which might eventually impact query performance.
Before changing it, please also take a look at related MergeTree settings, such as [number_of_free_entries_in_pool_to_lower_max_size_of_merge](../../operations/settings/merge-tree-settings.md#number-of-free-entries-in-pool-to-lower-max-size-of-merge) and [number_of_free_entries_in_pool_to_execute_mutation](../../operations/settings/merge-tree-settings.md#number-of-free-entries-in-pool-to-execute-mutation).
Sets a ratio between the number of threads and the number of background merges and mutations that can be executed concurrently. For example, if the ratio equals to 2 and
`background_pool_size` is set to 16 then ClickHouse can execute 32 background merges concurrently. This is possible, because background operations could be suspended and postponed. This is needed to give small merges more execution priority. You can only increase this ratio at runtime. To lower it you have to restart the server.
The same as for `background_pool_size` setting `background_merges_mutations_concurrency_ratio` could be applied from the `default` profile for backward compatibility.
Sets the limit on how much RAM is allowed to use for performing merge and mutation operations.
Zero means unlimited.
If ClickHouse reaches this limit, it won't schedule any new background merge or mutation operations but will continue to execute already scheduled tasks.
It is enabled by default. If it`s not, you can do this manually.
**Enabling**
To manually turn on metrics history collection [`system.metric_log`](../../operations/system-tables/metric_log.md), create `/etc/clickhouse-server/config.d/metric_log.xml` with the following content:
To disable `metric_log` setting, you should create the following file `/etc/clickhouse-server/config.d/disable_metric_log.xml` with the following content:
Support for SSL is provided by the `libpoco` library. The available configuration options are explained in [SSLManager.h](https://github.com/ClickHouse-Extras/poco/blob/master/NetSSL_OpenSSL/include/Poco/Net/SSLManager.h). Default values can be found in [SSLManager.cpp](https://github.com/ClickHouse-Extras/poco/blob/master/NetSSL_OpenSSL/src/SSLManager.cpp).
- privateKeyFile – The path to the file with the secret key of the PEM certificate. The file may contain a key and certificate at the same time.
- certificateFile – The path to the client/server certificate file in PEM format. You can omit it if `privateKeyFile` contains the certificate.
- caConfig (default: none) – The path to the file or directory that contains trusted CA certificates. If this points to a file, it must be in PEM format and can contain several CA certificates. If this points to a directory, it must contain one .pem file per CA certificate. The filenames are looked up by the CA subject name hash value. Details can be found in the man page of [SSL_CTX_load_verify_locations](https://www.openssl.org/docs/man3.0/man3/SSL_CTX_load_verify_locations.html).
- verificationMode (default: relaxed) – The method for checking the node’s certificates. Details are in the description of the [Context](https://github.com/ClickHouse-Extras/poco/blob/master/NetSSL_OpenSSL/include/Poco/Net/Context.h) class. Possible values: `none`, `relaxed`, `strict`, `once`.
- verificationDepth (default: 9) – The maximum length of the verification chain. Verification will fail if the certificate chain length exceeds the set value.
- loadDefaultCAFile (default: true) – Wether built-in CA certificates for OpenSSL will be used. ClickHouse assumes that builtin CA certificates are in the file `/etc/ssl/cert.pem` (resp. the directory `/etc/ssl/certs`) or in file (resp. directory) specified by the environment variable `SSL_CERT_FILE` (resp. `SSL_CERT_DIR`).
- cacheSessions (default: false) – Enables or disables caching sessions. Must be used in combination with `sessionIdContext`. Acceptable values: `true`, `false`.
- sessionIdContext (default: `${application.name}`) – A unique set of random characters that the server appends to each generated identifier. The length of the string must not exceed `SSL_MAX_SSL_SESSION_ID_LENGTH`. This parameter is always recommended since it helps avoid problems both if the server caches the session and if the client requested caching. Default value: `${application.name}`.
- sessionCacheSize (default: [1024\*20](https://github.com/ClickHouse/boringssl/blob/master/include/openssl/ssl.h#L1978)) – The maximum number of sessions that the server caches. A value of 0 means unlimited sessions.
- sessionTimeout (default: [2h](https://github.com/ClickHouse/boringssl/blob/master/include/openssl/ssl.h#L1926)) – Time for caching the session on the server.
- extendedVerification (default: false) – If enabled, verify that the certificate CN or SAN matches the peer hostname.
- fips (default: false) – Activates OpenSSL FIPS mode. Supported if the library’s OpenSSL version supports FIPS.
- privateKeyPassphraseHandler (default: `KeyConsoleHandler`)– Class (PrivateKeyPassphraseHandler subclass) that requests the passphrase for accessing the private key. For example: `<privateKeyPassphraseHandler>`, `<name>KeyFileHandler</name>`, `<options><password>test</password></options>`, `</privateKeyPassphraseHandler>`.
- invalidCertificateHandler (default: `ConsoleCertificateHandler`) – Class (a subclass of CertificateHandler) for verifying invalid certificates. For example: `<invalidCertificateHandler> <name>ConsoleCertificateHandler</name> </invalidCertificateHandler>` .
- disableProtocols (default: "") – Protocols that are not allowed to use.
- preferServerCiphers (default: false) – Preferred server ciphers on the client.
Logging events that are associated with [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md). For instance, adding or merging data. You can use the log to simulate merge algorithms and compare their characteristics. You can visualize the merge process.
Queries are logged in the [system.part_log](../../operations/system-tables/part_log.md#system_tables-part-log) table, not in a separate file. You can configure the name of this table in the `table` parameter (see below).
-`partition_by` — [Custom partitioning key](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) for a system table. Can't be used if `engine` defined.
-`order_by` - [Custom sorting key](../../engines/table-engines/mergetree-family/mergetree.md#order_by) for a system table. Can't be used if `engine` defined.
-`engine` - [MergeTree Engine Definition](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-creating-a-table) for a system table. Can't be used if `partition_by` or `order_by` defined.
-`flush_interval_milliseconds`– Interval for flushing data from the buffer in memory to the table.
-`storage_policy`– Name of storage policy to use for the table (optional)
-`settings` - [Additional parameters](../../engines/table-engines/mergetree-family/mergetree.md/#settings) that control the behavior of the MergeTree (optional).
-`endpoint`– HTTP endpoint for scraping metrics by prometheus server. Start from ‘/’.
-`port`– Port for `endpoint`.
-`metrics`– Flag that sets to expose metrics from the [system.metrics](../../operations/system-tables/metrics.md#system_tables-metrics) table.
-`events`– Flag that sets to expose metrics from the [system.events](../../operations/system-tables/events.md#system_tables-events) table.
-`asynchronous_metrics`– Flag that sets to expose current metrics values from the [system.asynchronous_metrics](../../operations/system-tables/asynchronous_metrics.md#system_tables-asynchronous_metrics) table.
Queries are logged in the [system.query_log](../../operations/system-tables/query_log.md#system_tables-query_log) table, not in a separate file. You can change the name of the table in the `table` parameter (see below).
-`table`– Name of the system table the queries will be logged in.
-`partition_by` — [Custom partitioning key](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) for a system table. Can't be used if `engine` defined.
-`order_by` - [Custom sorting key](../../engines/table-engines/mergetree-family/mergetree.md#order_by) for a system table. Can't be used if `engine` defined.
-`engine` - [MergeTree Engine Definition](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-creating-a-table) for a system table. Can't be used if `partition_by` or `order_by` defined.
-`flush_interval_milliseconds`– Interval for flushing data from the buffer in memory to the table.
-`storage_policy`– Name of storage policy to use for the table (optional)
-`settings` - [Additional parameters](../../engines/table-engines/mergetree-family/mergetree.md/#settings) that control the behavior of the MergeTree (optional).
If the table does not exist, ClickHouse will create it. If the structure of the query log changed when the ClickHouse server was updated, the table with the old structure is renamed, and a new table is created automatically.
Data for the query cache is allocated in DRAM. If memory is scarce, make sure to set a small value for `max_size_in_bytes` or disable the query cache altogether.
Setting for logging threads of queries received with the [log_query_threads=1](../../operations/settings/settings.md#settings-log-query-threads) setting.
Queries are logged in the [system.query_thread_log](../../operations/system-tables/query_thread_log.md#system_tables-query_thread_log) table, not in a separate file. You can change the name of the table in the `table` parameter (see below).
-`table`– Name of the system table the queries will be logged in.
-`partition_by` — [Custom partitioning key](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) for a system table. Can't be used if `engine` defined.
-`order_by` - [Custom sorting key](../../engines/table-engines/mergetree-family/mergetree.md#order_by) for a system table. Can't be used if `engine` defined.
-`engine` - [MergeTree Engine Definition](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-creating-a-table) for a system table. Can't be used if `partition_by` or `order_by` defined.
-`flush_interval_milliseconds`– Interval for flushing data from the buffer in memory to the table.
-`storage_policy`– Name of storage policy to use for the table (optional)
-`settings` - [Additional parameters](../../engines/table-engines/mergetree-family/mergetree.md/#settings) that control the behavior of the MergeTree (optional).
If the table does not exist, ClickHouse will create it. If the structure of the query thread log changed when the ClickHouse server was updated, the table with the old structure is renamed, and a new table is created automatically.
Setting for logging views (live, materialized etc) dependant of queries received with the [log_query_views=1](../../operations/settings/settings.md#settings-log-query-views) setting.
Queries are logged in the [system.query_views_log](../../operations/system-tables/query_views_log.md#system_tables-query_views_log) table, not in a separate file. You can change the name of the table in the `table` parameter (see below).
-`table`– Name of the system table the queries will be logged in.
-`partition_by` — [Custom partitioning key](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) for a system table. Can't be used if `engine` defined.
-`order_by` - [Custom sorting key](../../engines/table-engines/mergetree-family/mergetree.md#order_by) for a system table. Can't be used if `engine` defined.
-`engine` - [MergeTree Engine Definition](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-creating-a-table) for a system table. Can't be used if `partition_by` or `order_by` defined.
-`flush_interval_milliseconds`– Interval for flushing data from the buffer in memory to the table.
-`storage_policy`– Name of storage policy to use for the table (optional)
-`settings` - [Additional parameters](../../engines/table-engines/mergetree-family/mergetree.md/#settings) that control the behavior of the MergeTree (optional).
If the table does not exist, ClickHouse will create it. If the structure of the query views log changed when the ClickHouse server was updated, the table with the old structure is renamed, and a new table is created automatically.
-`level` — Maximum Message Level (by default `Trace`) which will be stored in a table.
-`database` — Database name.
-`table` — Table name.
-`partition_by` — [Custom partitioning key](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) for a system table. Can't be used if `engine` defined.
-`order_by` - [Custom sorting key](../../engines/table-engines/mergetree-family/mergetree.md#order_by) for a system table. Can't be used if `engine` defined.
-`engine` - [MergeTree Engine Definition](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-creating-a-table) for a system table. Can't be used if `partition_by` or `order_by` defined.
-`flush_interval_milliseconds` — Interval for flushing data from the buffer in memory to the table.
-`storage_policy`– Name of storage policy to use for the table (optional)
-`settings` - [Additional parameters](../../engines/table-engines/mergetree-family/mergetree.md/#settings) that control the behavior of the MergeTree (optional).
-`partition_by` — [Custom partitioning key](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) for a system table. Can't be used if `engine` defined.
-`order_by` - [Custom sorting key](../../engines/table-engines/mergetree-family/mergetree.md#order_by) for a system table. Can't be used if `engine` defined.
-`engine` - [MergeTree Engine Definition](../../engines/table-engines/mergetree-family/index.md) for a system table. Can't be used if `partition_by` or `order_by` defined.
-`flush_interval_milliseconds` — Interval for flushing data from the buffer in memory to the table.
-`storage_policy`– Name of storage policy to use for the table (optional)
-`settings` - [Additional parameters](../../engines/table-engines/mergetree-family/mergetree.md/#settings) that control the behavior of the MergeTree (optional).
Settings for the [asynchronous_insert_log](../../operations/system-tables/asynchronous_insert_log.md#system_tables-asynchronous_insert_log) system table for logging async inserts.
Parameters:
-`database` — Database name.
-`table` — Table name.
-`partition_by` — [Custom partitioning key](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) for a system table. Can't be used if `engine` defined.
-`engine` - [MergeTree Engine Definition](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-creating-a-table) for a system table. Can't be used if `partition_by` defined.
-`flush_interval_milliseconds` — Interval for flushing data from the buffer in memory to the table.
-`storage_policy`– Name of storage policy to use for the table (optional)
Configuration of clusters used by the [Distributed](../../engines/table-engines/special/distributed.md) table engine and by the `cluster` table function.
The time zone is necessary for conversions between String and DateTime formats when DateTime fields are output to text format (printed on the screen or in a file), and when getting DateTime from a string. Besides, the time zone is used in functions that work with the time and date if they didn’t receive the time zone in the input parameters.
The directory with user scripts files. Used for Executable user defined functions [Executable User Defined Functions](../../sql-reference/functions/index.md#executable-user-defined-functions).
The directory with user defined files. Used for SQL user defined functions [SQL User Defined Functions](../../sql-reference/functions/index.md#user-defined-functions).
ClickHouse uses ZooKeeper for storing metadata of replicas when using replicated tables. If replicated tables are not used, this section of parameters can be omitted.
-`session_timeout_ms` — Maximum timeout for the client session in milliseconds.
-`operation_timeout_ms` — Maximum timeout for one operation in milliseconds.
-`root` — The [znode](http://zookeeper.apache.org/doc/r3.5.5/zookeeperOver.html#Nodes+and+ephemeral+nodes) that is used as the root for znodes used by the ClickHouse server. Optional.
-`identity` — User and password, that can be required by ZooKeeper to give access to requested znodes. Optional.
ClickHouse uses the setting for all the tables on the server. You can change the setting at any time. Existing tables change their behaviour when the setting changes.
When creating a table, specify the corresponding [engine setting](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-creating-a-table). The behaviour of an existing table with this setting does not change, even if the global setting changes.
If `use_minimalistic_part_header_in_zookeeper = 1`, then [replicated](../../engines/table-engines/mergetree-family/replication.md) tables store the headers of the data parts compactly using a single `znode`. If the table contains many columns, this storage method significantly reduces the volume of the data stored in Zookeeper.
After applying `use_minimalistic_part_header_in_zookeeper = 1`, you can’t downgrade the ClickHouse server to a version that does not support this setting. Be careful when upgrading ClickHouse on servers in a cluster. Don’t upgrade all the servers at once. It is safer to test new versions of ClickHouse in a test environment, or on just a few servers of a cluster.
The configurable settings within `<distributed_ddl>` include:
- **path**: the path in Keeper for the `task_queue` for DDL queries
- **profile**: the profile used to execute the DDL queries
- **pool_size**: how many `ON CLUSTER` queries can be run simultaneously
- **max_tasks_in_queue**: the maximum number of tasks that can be in the queue. Default is 1,000
- **task_max_lifetime**: delete node if its age is greater than this value. Default is `7 * 24 * 60 * 60` (a week in seconds)
- **cleanup_delay_period**: cleaning starts after new node event is received if the last cleaning wasn't made sooner than `cleanup_delay_period` seconds ago. Default is 60 seconds
If this section is specified, the path from [users_config](../../operations/server-configuration-parameters/settings.md#users-config) and [access_control_path](../../operations/server-configuration-parameters/settings.md#access_control_path) won't be used.
The `user_directories` section can contain any number of items, the order of the items means their precedence (the higher the item the higher the precedence).
You can also define sections `memory` — means storing information only in memory, without writing to disk, and `ldap` — means storing information on an LDAP server.
-`server` — one of LDAP server names defined in `ldap_servers` config section. This parameter is mandatory and cannot be empty.
-`roles` — section with a list of locally defined roles that will be assigned to each user retrieved from the LDAP server. If no roles are specified, user will not be able to perform any actions after authentication. If any of the listed roles is not defined locally at the time of authentication, the authentication attempt will fail as if the provided password was incorrect.
Sets the memory size (in bytes) for a stack trace at every peak allocation step. The data is stored in the [system.trace_log](../../operations/system-tables/trace_log.md) system table with `query_id` equal to an empty string.
Allows to collect random allocations and deallocations and writes them in the [system.trace_log](../../operations/system-tables/trace_log.md) system table with `trace_type` equal to a `MemorySample` with the specified probability. The probability is for every allocation or deallocations, regardless of the size of the allocation. Note that sampling happens only when the amount of untracked memory exceeds the untracked memory limit (default value is `4` MiB). It can be lowered if [total_memory_profiler_step](#total-memory-profiler-step) is lowered. You can set `total_memory_profiler_step` equal to `1` for extra fine-grained sampling.
